P993: INCREASED STAT5B LEVELS IN MYELOPROLIFERATIVE NEOPLASMS LEAD TO ENHANCED MEGAKARYOCYTIC DIFFERENTIATION CORRELATING WITH THROMBOSIS
Authors: Marie Ploderer, Barbara Maurer, Lena Schreiberhuber, Tobias Suske, Gabriela Feurstein, Jihyun Song, Josef Prchal, Richard Moriggl, Heidi Neubauer
Journal: HemaSphere (2023)

Topic: 15. Myeloproliferative neoplasms - Biology & Translational Research Background: Myeloproliferative neoplasms (MPNs) are a group of malignant diseases marked by the expansion of mature myeloid cells. The classical MPN subtypes include Polycythemia Vera (PV), Essential Thrombocythemia and Primary Myelofibrosis. The main cause of morbidity and mortality in MPN patients are thromboembolic events. The JAK2V617F mutation is found in a majority of MPN patients and is associated with an increased risk of thrombosis. Only few additional risk factors for thrombosis are known and not all are useful for risk stratification. Important downstream targets activated by JAK2V617F signalling include the two STAT5 proteins, STAT5A and STAT5B. Although increased expression levels of STAT5 have been shown to correlate with disease progression in BCR-ABL-driven Chronic Myeloid Leukemia, the consequence of higher STAT5 levels have not yet been investigated in classical MPNs. Aims: We aimed to investigate the effect of increased STAT5 levels on clinical parameters affecting survival in MPN patients, such as thrombosis. We also aimed to explore the impact of elevated STAT5 levels on MPN disease, using a JAK2V617F MPN mouse model with a low-level increase in STAT5 expression. Methods: STAT5 mRNA expression levels in granulocytes isolated from JAK2V617F-positive MPN patients were determined by RNAseq and correlated with the history of thromboembolic events. The stem cell compartment, megakaryocyte progenitors (MkPs) and mature myeloid cells of the transgenic mice were quantified by flow cytometry. Mature megakaryocytes (Mks) were quantified from histological sections. Mk differentiation experiments were performed ex vivo using murine hematopoietic stem cells. Results: We found that JAK2V617F-positive MPN patients with a history of thrombosis had increased levels of STAT5B compared to those with no previous thromboembolic events. The opposite was true for STAT5A, where patients who had suffered a thromboembolic event had lower levels of STAT5A compared to those with no thrombosis history. To examine the impact of increased STAT5B levels on MPN pathogenesis, we crossed a JAK2V617F+ PV mouse model with a transgenic mouse with a low-level increase in STAT5B levels. This increase in STAT5B expression led to decreased survival of the JAK2V617F+ mice. While most mature myeloid cell populations were unaffected, we observed a striking effect specifically on the Mk lineage, with increases in Mk-biased stem cells, MkPs and mature Mks. To investigate the effect of enhanced STAT5B on Mk differentiation, we performed ex vivo Mk differentiation experiments using stem cells isolated from the mice. Here, again, we saw that increased STAT5B levels enhanced Mk differentiation in JAK2V617F+ cells. Summary/Conclusion: Risk stratification for thrombosis in MPN patients is difficult and anticoagulant treatment used to lower thrombosis risk can lead to severe bleeding side-effects. Hence, a better understanding of thrombosis risk is essential to improve treatment of MPN patients. In this study, we observed that increased STAT5B levels correlated with a history of thrombosis in JAK2V617F-positive patients and led to expansion of the Mk lineage in an MPN model harbouring the JAK2V617F mutation. This increase in Mks could increase platelet levels, which are crucial in the development of thrombosis. We are therefore currently quantifying platelet levels and their activation as well as thrombotic events in these mice. We believe that STAT5B expression levels could increase thrombosis formation and be a novel prognostic marker for thrombosis in MPN. Keywords: Megakaryocyte, STAT5, Thrombosis, Myeloproliferative disorder